As a bearing which must be corrosion-resistant there has been heretofore used relatively often a sliding bearing made of a material having an excellent corrosion resistance. In recent years, rolling bearings have been used more and more from the standpoint of torque reduction that prevents dynamic loss or eliminates the necessity of maintenance and improvement of product quality.
As the material for such rolling bearings there is mostly used a low-alloy steel such as two kinds of high carbon chromium bearing steels (SUJ2) and case hardening steel (SCR420). However, rolling bearings are used in various working conditions. Thus, if such a rolling bearing made of a low-alloy steel is used under environmental conditions which can be contaminated by water content or sea water, the contamination by even a slight amount of water content or sea water corrodes the bearing portion thereof corrodes with rust that disables the rolling bearing from working. Thus, martensite stainless steel having an excellent corrosion resistance and a high chromium content (e.g., SUS440C) is used under such environmental conditions.
However, a rolling bearing comprising races and rolling elements both of which are made of martensite-based stainless steel (hereinafter simply referred to as "stainless steel") can exhibit an insufficient corrosion resistance in some working atmospheres. In this case, corrosion occurs with chromium-deficient layer in the vicinity of coarse eutectic carbide as a starting point to reduce precision such as surface smoothness, possibly making it impossible to secure the desired bearing life. In particular, a rolling bearing adapted for use in semiconductor producing apparatus, etc. is subject to attack by a corrosive gas or chemical that can corrode stainless steel. Thus, it is required that such a rolling bearing comprise a material having a better corrosion resistance than stainless steel.
From this standpoint of view, as a bearing material constituting a rolling bearing adapted for use in corrosive working atmospheres there has heretofore been used a ceramic material such as silicon nitride (Si.sub.3 N.sub.4) (hereinafter referred to as "first conventional technique").
In the machine tool industry, on the other hand, the recent trend is for more machines to operate at higher rotary speed. To this end, it is required for the rolling bearing for supporting the rotary portion of machine tools to have higher precision and withstand severer working conditions. When a machine tool operates at a raised rotary speed, the so-called bearing clearance is reduced, causing further rolling friction that adds to heat generation. As a result, the temperature of the bearing rises.
The rise in the heat generation due to rolling friction is considered to be attributed to the rise in the centrifugal force applied to the rolling elements. In order to lessen the centrifugal force and hence lower the temperature of the rolling elements, a rolling bearing comprising rolling elements made of ceramic material, which exhibit a small density (specific gravity), rather than low-alloy steel has heretofore been put into practical use. However, with the recent trend for more machine tools to operate at even higher rotary speed, mere reduction of the weight of the rolling elements cannot prevent the rise in the bearing temperature.
By the way, the heat generated in the outer race during high speed rotation normally is radiated to the exterior through the housing. Since the heat generated in the inner race can be difficultly radiated from the rotary axis, the temperature of the inner race is higher than that of the outer race. Thus, if the outer race and the inner race are formed by the same material, and the temperature of the inner race is raised by heat generation, the inner race undergoes a great thermal expansion that reduces the bearing clearance from the initial value. The resulting preload is excessive, accelerating the heat generation. This phenomenon occurs in a vicious circle. Eventually, the bearing undergoes seizing that can lead to the destruction of the bearing.
From this standpoint of view, a rolling bearing has been proposed comprising an inner race formed by a material having a smaller linear expansion coefficient than the outer race material (see JP-B-7-30788 (The term "JP-B" as used herein means an "examined Japanese patent publication")) (hereinafter referred to as "second conventional technique"). In accordance with the foregoing second conventional technique, the inner race is formed by a material having a smaller linear expansion coefficient than the outer race material. For example, the outer race may be formed by a high carbon chromium bearing steel (SUJ2) while the inner race may be formed by a stainless steel (SUS440C) or ceramic material. In this arrangement, even if the temperature of the inner race is higher than that of the outer race, the expansion of the inner race caused by the temperature difference between the inner race and the outer race can be inhibited. As a result, the variation of preload accompanying the change in the bearing clearance is reduced, making it possible to prevent the bearing from seizing.
A titanium alloy has a lighter weight and a higher strength than a steel material and a very excellent corrosion resistance among metallic materials and thus is expected to be a bearing material for use in special corrosive atmospheres such as those contaminated by water content, sea water, chemical, etc.
In a rolling bearing, however, a very great face pressure is applied to the portion at which the races and the rolling elements come in contact with each other. Thus, it is required for a rolling bearing to exhibit a high surface hardness. However, a titanium alloy which has been merely subjected to ordinary heat treatment such as solution treatment and aging cannot be provided with a desired surface hardness.
From this standpoint of view, a technique for enhancing the surface hardness of a titanium alloy by a predetermined surface treatment has been proposed (JP-B-61-2747) (hereinafter referred to as "third conventional technique").
In the foregoing third conventional technique, a titanium alloy is subjected to gaseous nitriding or carburizing so that penetrating elements such as C, N and O are diffused in the form of solid solution therein, thereby securing the surface hardness required for the races.
In the foregoing first conventional technique, a ceramic material is used as bearing material. Thus, the bearing exhibits an extremely good corrosion resistance as compared with stainless steel. However, the first conventional technique is disadvantageous in that a ceramic material is inferior to stainless steel in strength or toughness and thus cannot be used without any trouble in atmospheres subject to great load. In particular, the use of ceramic material as the race material is undesirable from the standpoint of reliability of bearing.
Further, a ceramic material is remarkably inferior to metallic material in formability and grindability. Thus, if all the essential parts of a bearing are formed by a ceramic material, it disadvantageously adds to the production cost.
Moreover, a ceramic material has an extremely smaller linear expansion coefficient than a metallic material. Thus, the foregoing conventional technique has some disadvantages. For example, if the outer race is formed by the foregoing high carbon chromium steel (SUJ2) and the inner race is formed by a ceramic material, the difference in thermal expansion between the metallic rotary axis and the inner race made of ceramic material becomes too great when the temperature rises to relax the thermal expansion of the rotary axis, possibly cracking the inner race made of ceramic material and hence causing the destruction of the bearing.
On the other hand, if the outer race is formed by a high carbon chromium bearing steel (SUJ2) and the inner race is formed by a stainless steel (SUS440C), the change in the bearing clearance caused by the temperature rise can be minimized because the linear expansion coefficient of stainless steel is as small as 80% of that of high carbon chromium bearing steel. Further, since a stainless steel is a metallic material, the inner race made of stainless steel is considered to be insusceptible to cracking due to the difference in thermal expansion between the rotary axis and the inner race unlike the inner race made of ceramic material.
However, since the stainless steel used as inner race material has a higher density (higher specific gravity) than the ceramic material, the rise in the centrifugal force applied to the inner race cannot be neglected. In other words, since centrifugal force increases in proportion to mass and speed, the inner race expands due to the centrifugal force produced by rotation as the rotary speed increases. As a result, the bearing clearance is reduced, accelerating the heat generation.
The foregoing third conventional technique is disadvantageous in that the resulting surface hardness and depth of hardening differ greatly with the kind of penetrating elements to be incorporated in the form of solid solution by surface treatment. Further, some titanium alloys used have too low a strength in the core to fulfill a sufficient function as bearing.
In accordance with the third conventional technique, the surface hardness of the titanium alloy can be enhanced by diffusing penetrating elements in the titanium alloy in the form of solid solution. However, these penetrating elements can embrittle the titanium alloy, making it impossible to obtain a desired bearing life.